Human monocytes have been employed in vitro assay systems to determine the macromolecular basis for their function. Monocytes have been studied in their unactivated state, following muramyl dipeptide activation, and following activation with poly ICLC; only the poly ICLC stimulated cells are capable of releasing interferon, whereas the other activation states of monocytes are characterized by possessing other distinctive functions. When total RNA metabolism and total messenger RNA synthesis was analyzed in these three distinct activation states of human monocytes, it was found that only monocytes stimulated with muramyl dipeptide displayed a reproducible increase in RNA metabolism; this increase in total and messenger RNA synethesis peaked at 4 hrs. following muramyl dipeptide activation. To evaluate whether the poly ICLC stimulated monocytes were indeed secreting interferon specific messenger RNA, Northern blotting analysis of messenger RNA obtained from these three monocyte activation states was performed with a cDNA probe for alpha interferon. It was found that poly ICLC stimulated monocytes synthesized three molecular forms of the alpha interferon message whereas unstimulated monocytes do not synthesize detectable amounts of alpha interferon messenger RNA. Muramyl dipeptide stimulated monocytes synthesize only a high molecular weight form of the interferon messenger RNA; synthesis of this message appears to be associated with an intracytoplasmic form of alpha interferon activity. This molecular biology technology has potential clinical application for monitoring the gene expression events associated with cancer immuotherapies operative on the blood monocytes of cancer patients.